In recent years, in certain applications optical fibers have become the preferred medium over copper wire for telecommunications, particularly high speed communication and data transmission. There are already millions of miles of optical fiber in use today, for both long distance hauls, and local distribution within a facility or building. Field installation, service and repair of optical fiber systems can be a delicate, time consuming and often troublesome procedure due to the fragile nature of the components involved, especially the optical fiber itself. Optical fibers are typically made of a material such as quartz, multi-component glass or synthetic resins and in view of their generally small diameter, such fibers are susceptible to high stresses when undergoing a force exerted in a direction at right angles to the fiber axis. For example, optical fibers made of quartz or multi-component glass are liable to break, and those made of synthetic resins are liable to bend or break under such a force. Even a slight bend (microbend) in an optical fiber can result in serious light leakage and consequent signal loss, and small deformations can induce fractures which over time propagate into large cracks.
A composite fiber optic cable typically includes an outer jacket, an inner buffer and a central or axial clad optical fiber or fibers. The outer jacket and the inner buffer are usually fabricated of flexible, tubular plastic material. Strength members, such as elongated strands of aramid fibers (e.g., KEVLAR), may be incorporated between the outer jacket and the inner buffer to protect the fiber and provide longitudinal strength for the cable while permitting easy manual manipulation of the cable itself. A single fiber optic cable can carry multiple fiber optic cores, or multi-fiber ribbons.
In order to terminate an optical fiber or to assemble the cable/fiber in a connector or splice, the outer jacket and inner buffer normally are removed to expose a length of the small brittle optical fiber therein. In terminating such a fiber within an optical fiber connector, for example, the connector often includes a ferrule, such as ceramic material, having a small center bore through which the fiber extends and barely protrudes from a distal end thereof for connection or mating with the fiber of a complementary connector.
There is a wide range of cable designs and methods of manufacture which provide various constructions of the outer jacket and inner tubing. In U.S. Pat. No. 4,172,106, the cable has a protective sheath that surrounds an optical fiber core having one or more optical fiber waveguides. The protective sheath consists of a plastic tube incorporating reinforcing tensile material in its wall, such as steel wires embedded in the sheath wall. The sheath is formed in a single continuous extrusion step.
U.S. Pat. No. 4,389,087 describes an optical fiber cable design that imparts mechanical protection and reinforcement. The optical fiber core is surrounded by two concentric (inner and outer) coverings. The inner covering is formed by extruding about the core a material having a high modulus of elasticity, such as a polyamide, polyethylene terephthalate, or a high density polyethylene. The outer covering, also made via extrusion, is formed of a material having a low modulus of elasticity, such as low-density polyethylene, polyvinyl chloride, or an ethylene and vinyl acetate copolymer. A mechanical supporting element in the form of a cord is disposed along the optical fiber between the inner and outer coverings.
U.S. Pat. No. 4,595,793 describes a flame-resistant plenum cable that may be used for a single optical fiber core or for a core having multiple fiber ribbons. The core is surrounded first by a fire-retardant plastic jacket. The plastic jacket is further surrounded by a sheath system having an inner layer of heat-resistant fibrous material. This inner layer is enclosed by a woven glass layer that is impregnated with a fluorocarbon resin. The outermost layer is a fluoropolymer plastic jacket. A similar design is seen in U.S. Pat. No. 4,605,818.
In U.S. Pat. No. 4,740,053, a cable for optical fibers comprises a compound sheath having inner and outer layers. The inner layer is high modulus and density, while the outer layer is low density. Both layers are formed by extrusion. The outer layer is constructed of foamed polyethylene, and the inner layer is constructed of polypropylene.
U.S. Pat. No. 4,781,433 shows another optical fiber plenum cable having a core that may include a plurality of buffer-coated optical fibers. The optical fibers are individually or collectively enclosed in a fibrous strength member. An outer jacket is provided over the strength member, comprising a plastic material with resistance to flame spread and smoke evolution.
U.S. Pat. No. 5,189,721 depicts an optical fiber ribbon cable with a series of clad optical fibers aligned in a row. The optical fibers are surrounded by a layer of porous expanded polytetrafluoroethylene tape that has coatings of adhesive on both sides. Applied over the double-coated tape is another tape made of polyester and coated again with adhesive on at least its inner side adjacent the first tape. The polyester tape is surrounded by a braided sheath of aramid fibers. A binder ribbon is then wrapped around the braided sheath. The cable is completed by extruding an outer jacket of flame-retardant polyvinyl chloride.
U.S. Pat. No. 5,561,731 describes a flexible casing for optical fiber ribbons which is rectangular in cross-section. The casing has an inner tube made of low friction material, and an outer tube made of polyvinyl chloride, with flexible tensile fibers located between the inner and outer tubes.
U.S. Pat. No. 6,061,488 describes a fiber optic cable adapted for more difficult environments. A single optical fiber or a bundle of optical fibers extend inside a metal tube. A braided ceramic sheath is interposed between the fiber(s) and the metal tube. The metal tube serves to hold the fibers in place by compressing the sheath.
In European Patent No. 1,031,864, a data transmission cable has a core element enclosed by an extruded polymer sheath consisting of two or more polymers with low adhesion to one another. One layer is exclusively or predominantly made of one particular polymer, and another layer is made of a mixture of the polymers. The sheath provides a mechanically strong exterior and a flame-resistant interior, while being easy to strip from the cable.
While there are accordingly many different designs for fiber optic cable constructions, these constructions require heavy jacketing materials to be placed about the optical fibers. Although heavy jacket materials serve the purpose of protecting the cables as they are routed through chases and plenums during the installation, their use creates a secondary problem wherein the flexibility of the cables is limited by the stiffness of the jacket. The stiffness of the outer jacket may prevent convenient routing of these cables to the back plane of a cabinet or face panel. Additionally, the large diameters of these heavy jackets may prevent tight radius routing and mechanical mating of these cables to industry standard connectors. If the outer protective jacket is removed to allow more flexible handling of a terminal portion of the cable, there is insufficient physical protection for this terminal portion. Moreover, the inner layers of prior art cables have little or no flame retardancy.
In light of the foregoing, it would be desirable to devise an improved fiber optic cable construction.